In a direct-sequence (DS) code-division-multiple-access (CDMA) system having a base station and a plurality of remote stations transmitting to the base station, the spread-spectrum signals from many of the remote stations arrive at the base station simultaneously. The spread-spectrum signal from each remote station may arrive at the base station with a different power level with different symbol and chip arrival times. Further, the desired spread-spectrum signal at a particular spread-spectrum receiver receiving a particular spread-spectrum channel from a particular remote station, may be fading, and is, on occasion, not detectable, or has a high error rate.
Diversity coding, forward-error-correction (FEC) decoding, and interference cancellation are approaches to reducing the error rates. RAKE may be used to combine the strongest signal paths in a fading or multipath environment. These approaches do not, in general, take advantage of the unique noise environment of a DS-CDMA system, in which noise, on the average, is due to the multiple spread-spectrum signals from the plurality of remote stations.
A general object of the invention is to reduce error rate in a direct-sequence code-division-multiple-access (DS-CDMA) spread-spectrum system.
Another object of the invention is to use the noise interference from the multiple users in the DS-CDMA system as side information in reducing error rate for decoding differentially-encoded data.
According to the present invention, as embodied and broadly described herein, an improvement to a spread-spectrum receiver at the base station in a direct-sequence code-division-multiple-access (DS-CDMA) system is provided. The spread-spectrum receiver, in a DS-CDMA system has, at an input, a plurality of spread-spectrum signals, arriving from a plurality of remote users, respectively. Each spread-spectrum signal in the plurality of spread-spectrum signals has a differentially encoded-data symbol. Each differentially encoded-data symbol is spread-spectrum processed by a chip-sequence signal lasting a symbol time TS. Each remote user may be operating at a different symbol time Tsi, where i is an index for the different symbol time. Each chip-sequence signal in the plurality of chip-sequence signals is different, due to a different chip sequence, from other chip-sequence signals used by other spread-spectrum signals in the plurality of spread-spectrum signals.
Each spread-spectrum receiver in the base station includes a matched filter having an impulse response matched to a desired chip-sequence signal in the plurality of chip-sequence signals. The matched filter detects a desired spread-spectrum signal in the plurality of spread-spectrum signals arriving at the spread-spectrum receiver at the base station. The desired spread-spectrum signal is spread-spectrum processed with a desired chip-sequence signal.
The improvement comprises a symbol sampler, a noise sampler, a relative-signal-level decoder, an estimator, an erasure detector, and an erasure decoder. The symbol sampler samples at a plurality of symbol times nTS, a plurality of symbol samples from the desired matched filter. The integer n indexes the plurality of symbol times. Each symbol sample has time duration TS. The relative-signal-level decoder decodes, with reference to the relative-signal-level of the current and previously received symbol samples, the plurality of symbol samples, thereby generating a plurality of decoded-symbol samples. As a result of noise and interference, these samples are non-binary. Hard limiting these samples prior to processing is not a preferred embodiment, but is an option included herein.
The noise sampler samples before, after, or a combination of before and after each symbol sample at a plurality of chip times kTC, but not at the symbol time TS, a plurality of noise samples. The estimator processes the plurality of noise samples to generate a noise estimate. The erasure detector detects, for each symbol sample and from the noise estimate, an erasure condition, and thereby generates an erasure signal. In response to the data and the erasure signals, the erasure FEC decoder, erasure decodes the symbols, as is well known in the art.
Additional objects and advantages of the invention are set forth in part in the description which follows, and in part are obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention also may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.